The power requirements for electrical and electronic systems being designed today are placing increasing demands upon power supply designs. The latest semiconductor devices call for lower and lower supply voltage levels. The typical 5 V supply has been reduced to 3.3 V for many components. Many semiconductor devices already available require an even lower supply voltage of 2.8 V such as memory devices.
A high level block diagram illustrating an example of a prior art power supply distribution system in an electronic device is shown in FIG. 1. The power supply distribution system, generally referenced 200, comprises a power supply 202, power supply wires or cables 204, sense wires 206 for voltage feedback, distribution bus 208 and a plurality of printed circuit cards (PCBs) 210, 212.
The power supply 202 receives an input voltage from a source of electrical power and functions to generate an output voltage which is distributed to the power distribution bus 208 via cables 204. Cables 206 comprise sense wires to provide voltage feedback to the power supply 202. The power supply 202 utilizes the feedback voltage in maintaining a stable output voltage.
Typical systems comprise a plurality of PC boards that connect to a backplane via a modular connector. For example printed circuit board 210 connects to the power distribution bus, i.e., the typically the backplane, via connector 220. Similarly, printed circuit board 212 connects to the power distribution bus via connector 222.
On some printed circuit boards, also termed plug-in boards or modules, electrical power is delivered directly to the board once the board 210 is seated in the connector 220. The electrical load placed on the power supply 202 is represented by the load block 211 on printed circuit board 210.
On other printed circuit boards electrical power is switched on the board itself. Printed circuit board 212 is an example of such a type of board. After the board 212 is seated in the connector 222, electrical power flows to the load 213 only when switches 215 are closed. In this case, electrical power to the plug-in modules like module 213 is controlled by switching devices such as switches 215 on board 212. Typically, the unit housing the distribution system 200 comprises a central control unit (not shown) which functions to control electrical power to the modules. Once a new module is installed in the system, for example, a request is made to the central control unit to activate the new module. Upon receiving the request, the central control unit examines the functional parameters of the particular module and if the parameters are within predetermined tolerances, the central control unit switches on electrical power to the new plug-in module.
The switching device 215 may comprise any suitable switch such as an electromechanical relay, solid sate relay, transistor or other controllable switching device.
The prior art electrical power distribution scheme described above, however, fails to deliver electrical power with sufficient accuracy when the required voltage levels begins to drop, for example, to 3.3 V and less. This is a major disadvantage especially considering that, the current trend in electronic technology is to operate electronic components at lower and lower voltages, e.g., 3.3 V +/-5%, 2.8 V +/-5% or lower. At such low voltage values, the current needed to be supplied is fairly large while the permitted variability of the voltage supply is only a few tens of millivolts. Even a small modest impedance naturally existent in the copper traces and connectors making up the power distribution path will cause voltage drops much larger than tens of millivolts. To make matters worse, the impedance in the copper traces and the connectors is usually not a design parameter that can be adjusted arbitrarily. In actuality, the impedance in the copper trances and the connectors is typically unpredictable.
The following example illustrates the problems associated with the prior art power distribution system. Consider a plug-in module that consumes 100 W which at 3.3 V draws approximately 30 A. An impedance of 10 m.OMEGA. would generate a drop of approximately 300 mV. This voltage drop is already almost twice as large as the 5% tolerance of 165 mV. In another example, if one considers a power FET, the typical R.sub.DS (On) impedance is approximately 4 m.OMEGA.. A current of 40 A yields a voltage drop of 160 mV which almost equals the 3.3 V 5% tolerance. Further, higher impedances, lower supply voltages and tighter tolerances only worsen the problem.